1. Field of the Invention
The present invention relates to a laser power control device, and more particularly, to a laser power control device for use in an optical recording and/or reproducing apparatus, which can accurately control peak power of a laser diode.
2. Description of the Related Art
In general, a laser diode is used as a light source for the optical pick-up device of an optical recording and/or reproducing apparatus such as a CD player, a recordable CD (CD-R) drive, or a rewritable CD (CD-RW) drive. To assure the smooth operation of the optical recording and/or reproducing apparatus, the power of the laser diode must be maintained at a specified level. Control over the power of the laser diode is needed, since the power of the laser diode may significantly change with changes in the ambient temperature or aging of the laser diode.
In order to control the power of the laser diode, the difference between a present power of the laser diode and a desired power of the laser diode has to be compensated for. A photodiode is used to measure the present power of the laser diode, partially receiving laser light radiated from the laser diode. The photodiode generates a current proportional to the intensity of the received laser light. The generated current is converted into voltage by a monitoring circuit, which functions as a current-to-voltage conversion circuit and is connected to the photodiode. This voltage is used as an input reflecting the present power of the laser diode.
Based on the type of the optical disk, the laser diode uses one of a pulse train method and a single pulse method to radiate light to record data onto an optical disk.
FIGS. 1A-1C relate to the pulse train method used in a CD-RW and a rewritable DVD (DVD-RW) drive. In the pulse train method, the laser diode eliminates previously recorded data from the optical disk by radiating an elimination power Pe on each space part between the pits of the optical disk, and forms a pit in the mark part of the optical disk by alternatively radiating on the optical disk a maximum power, i.e., a peak power Pp, and a minimum power, i.e., a bias power Pb, at high speeds. The first peak interval of each marked part is longer than other intervals in order to appropriately heat dyes coated on the optical disk.
As shown in the waveform of FIG. 1B, (a waveform which represents the output of a monitoring circuit ) the output waveform from the mark part does not accurately keep up with the output power waveform of the laser diode. This is because the output power of the laser diode is modulated at high speeds, while the response characteristic of the monitoring circuit is slower than the modulation of the laser diode.
FIGS. 2A-2B relates to the single pulse method used in a CD-R drive. In the single pulse method, the laser diode radiates bias power Pb on the space part, overdrive power Po on the initial part of the mark part, which helps to heat dyes coated on the optical disk appropriately, and write power Pw on the other part of the mark part, in which Pw<Po. According to the single pulse method of FIGS. 2A-2B, the output waveform of the monitoring circuit keeps up with the output power waveform of the laser diode in the space part and the mark part, in contrast to the pulse train method. This is because the output power of the laser diode is not modulated at high speeds.
FIG. 3 is a block diagram of a conventional laser power control device that controls the power of the laser diode according to the pulse train method.
A laser diode 15 radiates the laser light onto the optical disk. A photodiode 1 partially receives the laser light radiated from the laser diode 15 and outputs a current proportional to the intensity of the received laser light. A monitoring circuit 2 converts the current output from the photodiode 1 into a voltage. A sample-and-hold circuit 3 samples and holds the output of the monitoring circuit 2. An analog-to-digital (A/D) conversion circuit 4 converts the output of the sample-and-hold circuit 3 into a digital output. A calculation circuit 5 processes the output of the A/D conversion circuit 4. Digital-to-analog (D/A) conversion circuits 6, 7, and 8 convert the output of the calculation circuit 5 into analog outputs. Current sources 9, 10, and 11 are controlled by the outputs of the D/A conversion circuits 6, 7, and 8, respectively. A switch 12 performs a switching operation between the current source 9 and the laser diode 15. A switch 13 performs a switching operation between the current source 10 and the laser diode 15. A switch 14 performs a switching operation between the current source 11 and the laser diode 15.
The current source 9 supplies a laser driving current corresponding to the bias power Pb, i.e., a bias power current Ib, to the laser diode 15. The current source 10 supplies a laser driving current corresponding to the elimination power Pe, i.e., an elimination power current Ie, to the laser diode 15. The current source 11 supplies a laser driving current corresponding to the peak power Pp, i.e., a peak power current Ip, to the laser diode 15.
As shown in FIG. 1, the laser diode 15 alternatively radiates the peak power Pp and the bias power Pb, during a short time interval to form the pit in the mark part of the optical recording medium. The peak power Pp is generated when the switch 14 is switched on, and the switches 12 and 13 are switched off. The bias power Pb is generated when the switch 12 is switched on, and the switches 13 and 14 are switched off. When the laser diode 15 radiates the elimination power Pe on each space part to eliminate recorded data, the switch 13 is switched on, and the switches 12 and 14 are switched off. The switching operations of the switches 12, 13, and 14 are performed in response to input of a control signal SW, which corresponds to the data recording signal input to the laser diode driving integrated circuit (IC) (not shown) of the laser diode 15.
Hereinafter, the operation of the laser power control device of FIG. 3 will be described.
The photodiode 1 partially receives the laser light radiated from the laser diode 15 and outputs a current proportional to the intensity of the received laser light. The monitoring circuit 2 converts the output current into a voltage.
The output signal of the monitoring circuit 2 is input to the sample-and-hold circuit 3 that operates according to the sample-and-hold control signal shown as the third waveform of FIG. 1. As a result, the A/D conversion circuit 4 can detect the level of the present elimination power Pe.
The A/D conversion circuit 4 converts the level of the elimination power Pe into a digital output. The calculation circuit 5 processes the digital output and provides digital outputs to the D/A (Ib) conversion circuit 6, the D/A (Ie) conversion circuit 7, and the D/A (Ip) conversion circuit 8. Thus, the bias power current Ib, the elimination power current Ie, and the peak power current Ip are set to control the bias power Pb, the elimination power Pe, and the peak power Pp.
The D/A (Ib) conversion circuit 6, the D/A (Ie) conversion circuit 7, and the D/A (Ip) conversion circuit 8 control the bias power Pb, the elimination power Pe, and the peak power Pp as follows. The A/D conversion circuit 4 detects the level of the elimination power Pe output from the sample-and-hold circuit 3 and sends output to the calculation circuit 5. Considering the detected digital value of the elimination power Pe, the calculation circuit 5 provides a new digital value to the D/A (Ie) conversion circuit 7 to increase or decrease the output of the D/A (Ie) conversion circuit 7 to generate a desired elimination power. Also, the calculation circuit 5 provides digital output to the D/A (Ib) conversion circuit 6 and the D/A (Ip) conversion circuit 8. The digital output for each of these circuits is obtained by multiplying the digital output provided to the D/A (Ie) conversion circuit 7 by a number. The digital output to be provided to the D/A (Ib) conversion circuit 6 is obtained by multiplying the digital output provided to the D/A (Ie) conversion circuit 7 by a value Pb/Pe. The digital output provided to the D/A (Ip) conversion circuit 8 is obtained by multiplying the digital value provided to the D/A (Ie) conversion circuit 7 by a value Pp/Pe.
However, the conventional laser power control device has problems because, in addition to elimination power current (Ie), bias power (Pb) and peak power (Pp) are also controlled based on the detected level of elimination power (Pe).
Since the detected level of elimination power (Pe) may reflect noise, and thus, a value different from the original level of elimination power (Pe) may be input to the calculation circuit 5. In this case, the digital output provided to the D/A (Ie) conversion circuit 7 and the elimination power current (Ie) generated from the current source 10 may be erroneous. In the case described above, elimination power (Pe) is not significantly affected because noise is minimal. Yet, the digital output provided to the D/A (Ip) conversion circuit 8, obtained by multiplying the digital value provided to the D/A (Ie) conversion circuit 7 by the value Pp/Pe, may have an error multiplied by the value Pp/Pe. As a result, this error significantly affects the elimination power Pe, which prevents normal control of the peak power Pp.
In the conventional laser power control device, it is assumed that the current sources 10 and 11 have the same characteristic. That is, if the inputs to the current sources 10 and 11 are the same, the outputs thereof are assumed to be the same. In other words, it is assumed that if a current, obtained by multiplying the input to the current source 10 by the value Pp/Pe, is provided to the current source 11, peak power Pp, which is Pp/Pe times the elimination power Pe, is obtained. However, variations among the current sources 9, 10, and 11 exist, such that the current sources 9, 10, and 11 output different currents even if the same input is provided thereto. As a result, even if the input to the current source 10 multiplied by the value Pp/Pe is input to the current source 11, the ratio of the peak power Pp output from the current source 11 is not equal to the input elimination power Pe, which prevents normal control of the peak power Pp.
If no strict requirement exists for the range of the peak power Pp, the conventional laser power control device does not exhibit any problem. However, with the advent of new optical recording media such as DVD-RWs, the requirement for the range of the peak power Pp becomes stricter, and the conventional laser power control device cannot satisfy such strict requirements.